The principal aims of the proposed research are to delimit the major components of mating success in the housefly, Musca domestica L., and subsequently to examine how these components vary among selected populations of this species throughout the United States, and finally, to construct a holistic picture of the mating dynamics in the housefly through computer simulations. The basic experimental methodology is the direct observation technique of pair wise mating between all strains under competitive conditions in a plexiglass mating chamber, and then determining the separate contributions of sexual vigor and assortative mating to the observed deviations from random mating. The first year of the project revealed that male sexual vigor and assortative mating are the main behavioral components determining mating success and that these vary geographically in a way that can be meaningfully related to the ecology of the species. The current year is devoted to increasing our mating observations to other strains and expanding and testing our simulation model.